1. Field of the Invention
The invention relates generally to the field of electronic circuits, and, more particularly, to a circuit topology for filtering a radio frequency signal.
2. Art Background
Typically, receivers employ filter circuits to condition both input signals and internally generated reference signals. For example, band pass, notch, and low pass filters are types of filter circuits employed in such receivers. The frequency response of a filter refers to the characteristics of the filter that condition the signal input to the filter. For example, a band pass filter may attenuate an input signal across a predetermined band of frequencies above and below a center frequency of the filter. Filter circuits are designed to exhibit frequency responses based on one or more circuit parameters.
Some receivers are designed to process input signals within a range of input carrier frequencies (e.g., broadband receivers). For example, television receivers must be capable of processing input television signals with carrier frequencies ranging from 48 MHz to 890 MHz.
A popular application for filter circuits involves their use in television tuners. It is a conventional practice to mix an antenna signal with a local oscillator frequency for conversion to an intermediate frequency. However, prior to such mixing of signals, filters are necessary to filter out the useful signal band from the broadband reception signal.
Generally, mobile television requires small and thin television tuner modules. Thus, the filtering and mixing circuit blocks are usually incorporated on an integrated circuit (IC). Since the circuit blocks share the same substrate and need to reduce leakage to the substrate, an unbalanced to balanced filter is typically used within the television tuner.
FIG. 1 is a schematic diagram illustrating a prior art filter circuit to be used with a known television tuner module. As shown in FIG. 1, an unbalanced to balanced filter circuit 100 includes a transformer 110 coupled to a capacitor 120 having a variable capacitance C. The transformer 110 is an alternating current (AC) circuit, which further includes a primary coil 111 to receive a radio frequency input signal RFIN and a secondary coil 112, which are coupled to each other, for example, through a magnetic medium. The output terminations of the secondary coil 112 are coupled to the variable capacitor 120.
FIG. 2 is a graph illustrating various transfer functions for predetermined capacitance values of the variable capacitor 120 within the prior art filter circuit 100. As shown in FIG. 2, the transfer functions 200 correspond to different capacitance values of the variable capacitor 120, such as, for example, 4 picoFarad (pF), 8 pF, 16 pF, 32 pF, 64 pF, and 128 pF.
If the transformer 110 is a planar transformer, such as, for example, a 100 nanoHenry (nH) transformer, the thickness of the module needs to be maintained at a predetermined value, such as, for example, more than 1 millimeter, because the distance between the transformer 110 and a lid on the module should be maintained constant in order to minimize the loss. In addition, the total cost of the filter circuit 100 is relatively high due to the high cost of the planar transformer 110. Thus, what is needed is a filter circuit configuration, which is inexpensive and achieves a significant minimization of the module thickness and size.